Field of the Invention and Related Art Statement
The present invention generally relates to and electric blasting technique and more particularly to a method of electrically blasting a plurality of electric detonators provided at a plurality of working or exploding faces by centrally controlling a plurality of blasting devices arranged near respective working faces. This invention also relates to an apparatus for electrically blasting a plurality of electric detonators.
There has been proposed a method of electrically exploding a plurality of electric detonators which are electromagnetically coupled with a lead wire by means of transformer magnetic cores by supplying a pulsatory high frequency current to the lead wire. When the high frequency current is supplied to the lead wire, a secondary high frequency current is induced in a leg wire of an electric detonator. Then, a current flows through a fusehead of the detonator and the detonator is blasted. Such a method is described in U.S. Pat. No. 4,601,243 issued on July 22, 1986.
FIG. 1 is a schematic view illustrating this known method. To an electric blasting device 1 comprising an electric power source and an oscillator for generating a high frequency current, is connected a lead wire 2 having loop portions 2A with which transformer magnetic cores 3 are electromagnetically coupled. With the magnetic core 3 is further electromagnetically coupled loop-like wires 5 which are connected to fuseheads of detonators 4. When the pulsatory high frequency current is supplied from the electric blasting device 1 to the lead wire 2, a high frequency current is induced in each loop-like wire 5 via the magnetic core 3 by means of the electromagnetic induction. Then, the fusehead in the detonator 4 is heated to fire a detonating explosive.
In such a method, a pair of leg wires of the detonator are connected in the form of the loop wire 5, and thus leg wires are considered to be always short-circuited from the operation of coupling the loop wire 5 with the lead wire 2 via the magnetic core 3 to the actual exploding operation and the electric energy is hardly introduced into the loop wire. Therefore, any undesired explosion of the detonator can be effectively prevented.
In the known blasting method explained above, since the explosion is carried out at only a single working or blasting face with the aid of a single blasting device, when it is required to perform the explosion at a plurality of working faces, it is necessary to effect the explosion at working faces successively in time. Therefore, when the explosion has to be performed at a large number of locations such as in a mine of a large scale, difficult and complicated works are required in the warning of the explosion at different locations and the blasting time period is naturally prolonged so that the efficiency of blasting is very low. Further, care should be taken for ventilating the after-gas produced by the explosion. Moreover, the electric blasting device used in the known blasting method comprises both a power source unit for generating D.C. supply voltage and an oscillating unit for generating the high frequency pulsatory pulse in the same housing. Therefore, when the electric blasting device has to be placed at a location which is remote from the working face by, for instance, several kilometers, a very long lead wire must be used. In the electromagnetic induction type electric blasting method shown in FIG. 1, the impedance matching is effected in order to reduce a loss of the high frequency signal. Moreover, when use is made of the lead wire having the very long length up to several kilometers, the transmission loss could not be limited even by the impedance matching, so that it is rather difficult to transmit the sufficiently large electric energy for exploding the detonators positively. Therefore, in the known method, the length of the lead wire is limited to a relatively small distance such as several hundred meters.